A WIM sensor is a force transducer to be mounted in a roadway. By means of the WIM sensor inserted in the roadway, it is possible to determine a weight force of a vehicle traveling on the roadway. In the embodiment of a piezoelectric force transducer, a WIM sensor thus detects the weight force of a vehicle crossing the WIM sensor at a speed of up to 250 km/h with a sensitivity of several pC/N and with an accuracy of several percent. The weight force detected may be used to derive a broad range of traffic information such as wheel load, axle load, total weight, wheel pressure of the traveling vehicle.
A WIM sensor of the aforementioned type is disclosed in U.S. Pat. No. 5,265,481, which is hereby incorporated herein by this reference for all purposes. The WIM sensor comprises a cylindrical hollow profile made of metal having a diameter of 20 mm to 30 mm and a length of up to several meters. In the hollow profile are arranged a plurality of disc-shaped piezoelectric transducer elements. The WIM sensor is introduced in a groove of a depth of 30 mm to 50 mm in a surface layer of the roadway and cast with grout. The hollow profile has two functions, it protects the piezoelectric transducer elements from harmful environmental conditions and it directs a weight force to the piezoelectric transducer elements. Upon detecting a weight force, a piezoelectric transducer element emits an electrical signal that is proportional to the magnitude of the weight force detected and the emitted electrical signal is forwarded to an evaluation unit arranged outside of the hollow profile where the signal is evaluated. WIM sensor and evaluation unit constitute a system for automatic determination of traffic information.
However, the service life of a WIM sensor is longer than the service life of the surface layer of the roadway, especially on a roadway with intense traffic. Thus, if the surface layer of the roadway must be restored due to wear, then the WIM sensor inserted therein will be damaged and must be replaced.
Furthermore, also the WIM sensor itself placed in the surface layer of the roadway may be subject to wear, for example when the grout enclosing the WIM sensor is damaged and worn off locally as a result of adverse environmental conditions so that the WIM sensor is exposed to direct mechanical contact with traveling vehicles which adversely affects the service life of the WIM sensor.
Finally, the groove cast with grout differs in color from the surface layer of the road. This visually recognizable groove may distract and/or disconcert the drivers of the vehicles and cause dangerous evasive maneuvers which may be detrimental for traffic safety. In addition, the drivers of the vehicles may associate the telltale color difference in the surface of the roadway with the underlying presence of a WIM sensor that is mounted in a particular position and accordingly may perform dangerous evasive maneuvers that may be detrimental for traffic safety just to avoid detection of the weight force of their vehicle, for example in cases where a toll is charged depending on the vehicle weight force detected.